by "corrective" in this case you mean "a third blendshape that fixes a mix between two other blendshapes"? It's a pretty basic setup in essence, and the rest highly depends on how you want to rig faces in general. If you rebuild it all by script, where left/right side blendshapes are separated automatically as a final step of a setup, your tools will differ from the guy that assembles it all by hand.

If all you want is a shell button that creates corrective C for blends A and B, then all the button has to do is:
* take neutral shape
* apply blendshape to it with A, B, and corrected AB blend as targets; set A and B weights to -1 and AB to 1
* resulting shape is what needs to be plugged back to main blendshape node
* setup activation graph : weight(C) = weight(A)*weight(B)

Can you elaborate a bit more on the activation graph, I'm not quite getting what you're saying to do

Thank you

Also how do I make one shape dominant over another? Eg if I plug in UpperEyeClose and LowerEyeClose they'd clash, so when I use them both LowerEyeClose would be dialed down as UpperEyeClose is dialed up.

Originally Posted by JaredTaylor:Also how do I make one shape dominant over another? Eg if I plug in UpperEyeClose and LowerEyeClose they'd clash, so when I use them both LowerEyeClose would be dialed down as UpperEyeClose is dialed up.

I would not mix that together with corrective graph setup. Get your blends ready and mix well together, then when you're setting up control layer, deactivate lowerEye as upperEye is activated, which in pseudocode would be

lowerEyeWeight = lowerEyeControlValue*inverse(upperEyeWeight)

That's, obviously, if you just want what you wrote - "LowerEyeClose would be dialed down as UpperEyeClose is dialed up". Most probably what you want is lowerEyeClose dialed down as upper lid starts touching lower lid? It's then a much more complicated setup

Shorter explanation would be - the less automatic adjustments, the better. Trust your animator to animate what makes sense.

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